The purpose of this work is to study and further understand biochemical modulation of cancer chemotherapeutic agents in an in vivo situation. Carbon 14 and tritrium labeled deoxycytidine are used as tracers of pyrimidine salvage. Normal mice and L1210 bearing mice provide small intestine, bone marrow and spleen in which the selectivity of thymidine action on biochemical pathways is measured. HPLC techniques are used to measure nucleosides and nucleotides in these tissues and plasma. Extractions of lipids, RNA, DNA and protein, plus cesium sulfate gradients will permit analysis of incorporation of tracers into various tisssue components. These data will provide the means to determine kinetic rate constants for metabolism, transport and incorporation of various metabolites and the effects of thymidine on those rate constants. Deoxyazacytidine is being used as the antitumor agent in parallel studies on antitumor action in mice. Data are being collected on the degree of L1210 tumor cell kill and host toxicity following administration of this compound in combination with thymidine. The HPLC and tracer data provide clues on the optimal modes of administration of the combination of thymidine and deoxyazacytidine.